1. Field of the Invention:
This invention refers to steam turbine apparatus and in particular, to a flexible missile shield surrounding the turbine apparatus.
2. Description of the Prior Art:
If a large turbine generator power plant were to undergo an unexpected malfunction, such as a sudden loss of associated electrical load, while the turbines within the plant are operating at running speed, an overspeed condition is likely to occur. If the condition is not checked, the turbine could possibly fling itself apart, creating a large number of flying objects each having associated therewith a high kinetic energy. The safety problems created by the large numbers of flying objects emanating from the turbine are increased if the turbine which is destroyed is part of a nuclear steam power plant. The danger increases due to the potential hazards presented if one of the flying objects were to impinge upon and rupture any of the vital facilities associated with the nuclear side of the power plant, such as the waste fuel disposal storage piles located in nuclear power stations.
Of course, the primary method of insuring against such potential hazards is to effectively plan for each contingency condition which could arise, and taking steps to prevent any situation which would cause the turbine to literally fly apart. Also, planning the arrangement of the power plant in such a way as to minimize the accessibility of the number of vital targets likely to be struck by the high energy flying objects is another method of reducing the possibility of harm caused by turbine fragments. However, other prophylactic measures in addition to those mentioned are desirable, both from the standpoint of the power generating utility, which must meet certain safety requirements before construction or use permits are issued thereto, and from the Nuclear Regulatory Agency, which sets the applicable standards to be met before it issues any construction or use permits.
One such solution is to design the turbine casing itself in such a manner as to prevent the escape of flying objects therefrom. However, the casing structure which results when the casing is relied upon as a missile barrier has characteristics that seriously compromise the important functions of the turbine casing. In order for the casing-rotor-blade system of the turbine to efficiently and reliably generate power, the casing has the primary function first to contain and to guide required flows of steam through and around the blade path with minimum pressure losses, and second, to do this with a casing geometry having minimized thermal distortion and offering maximum resistance to thermal cyclic cracking. These functional requirements generally dictate minimizing wall thickness to the extent that they are compatible with pressure containment requirements.
However, analytical studies have shown that missile containment requires casing thicknesses of 10 to 15 times those presently required to satisfy the power generation function. For example, this guideline would require that the walls of the typical low pressure turbine casing, presently on the order of 1 to 11/2 inches thick, would increase to become within the range of 10 to 22 inches thick. Such increase thickness engenders much higher thermal stresses and distortions than occur in the present turbine casing construction, and in addition reduce casing life. Of course, other problems, such as manufacture and shipment of these more massive casings, plus stronger and more costly foundations also occur as a concomitant to reliance upon the casing as the primary missile barrier.
Another proposed solution is to dispose external to the turbine a thick, rigid, wall of steel or concrete enclosing the turbine apparatus to maintain flying objects within a predetermined volume about the apparatus. For a steel barrier, wall thickness on the order of 8 to 12 inches is necessary to provide an appropriate shield to maintain flying objects within a predetermined volume around the apparatus. One drawback with such a barrier design is the amount of weight that must be supported by the foundation and the support members keeping the barriers spaced away from the turbine. Another problem is the necessity of dismantling the barrier when periodic down-time inspection of the turbine are undertaken, with the attendant problems of handling such bulky and unwieldy missile barriers.
Although the prior art method of shielding has some disadvantages, it is evident that protection for sensitive components of the power plant must be provided to prevent any damage or potential hazards to personnel or property should a turbine apparatus be destroyed.